Recently, an “HSDPA (High Speed Downlink Packet Access) system” has been specified as a radio communication control system (see, for example, “3rd Generation Partnership Project” on the Internet <http://3gpp.org>). The HSDPA system is a higher speed packet transmission system for a downlink transmission in IMT-2000. The HSDPA system has purposes of achieving a higher peak transmission rate, a lower transmission delay, and a higher throughput and the like.
The HSPDA system is a transmission system for performing communications by sharing one physical channel among a plurality of mobile stations in a time-division. The HSDPA system allocates a channel to a mobile station of better radio quality at each moment. Thus, it is possible to improve the throughput of the entire system.
In order to perform communications by sharing one physical channel among a plurality of mobile stations in a time-division as described above, the mobile stations to perform communication by using the physical channel in each TTI (Transmission Time Interval) must be notified. In the case of the HSDPA system, the notification is made to the plurality of mobile stations by using a shared control channel called HS-SCCH (High Speed-Shared Control Channel).
In this regard, disclosed is a method in which a radio base station sets a transmission power to be allocated to the HS-SCCH, by adding an offset to an A-DPCH (Associated-Dedicated Physical Channel). The A-DPCH is dedicatedly set to each of the mobile stations (see, for example, Japanese Patent Application Laid-open Publication No. 2004-312530). The Japanese Patent Application Laid-open Publication No. 2004-312530 describes that the offset value is controlled in accordance with a communication quality (e.g., a block error rate) of the HS-SCCH.
However, in the control method described in Japanese Patent Application Laid-open Publication No. 2004-312530, the communication quality of the HS-SCCH cannot be obtained when a communication is started since the communication is yet to be performed. Moreover, since the TTI length and an encoding method differ significantly between the HS-SCCH and the A-DPCH, there has been a problem that an optimal value for the offset varies according to a propagation environment of the radio communication, for example, a fading frequency, the number of paths or an SIR.
In view of the above-mentioned problems, an object of the present invention is to provide a radio communication control system, a radio base station, and a radio communication control method that makes it possible to set a suitable transmission power of the HS-SCCH even when the communication is started, by setting the offset value in accordance with the propagation environment of the radio communication.